It is well known that the scattering of high energy radiation from a sample of material can yield information about the atomic structure of the material. Such radiation may be in the form of X-rays, gamma rays, cathode electrons, or the like. When a beam of radiation strikes a sample, a diffraction pattern of reflected radiation is created, which has a spatial intensity distribution that depends on the wavelength of the radiation and the type and position of the atoms in the material. If the sample is an oriented single crystal, the diffraction pattern consists of a pattrn of spots, corresponding to a projection of the reciprocal lattice of the crystal. Another important class of samples are randomly oriented--either polycrystalline, amorphous, or powdered--and the diffraction pattern becomes a series of cones, concentric with the incident beam. In each case, analysis of the position and intensity of the features of the diffraction pattern (spots or line segments), usually done by high speed digital computers, can reveal information about the atomic structure of the material. To achieve high quality information the location of the diffraction features must be accurately known, requiring adequate spatial resolution from the detector, and the intensity in the feature must be accurately determined. Since there is uncertainty in the statistics of the quanta contributing to a diffraction feature, accurate intensity determination requires both high enough exposures to give adequatee quatum noise and good siignal-to-noise ratio in the detector to not degrade this precision. For many diffraction pattrns it is also highly desirable to be able to gather data over as large an area as possible. The function of a diffraction detector is then to determine the posititon and the intensity of this diffraction pattern with adquate accuracy, and provide the digital data for analysis.
Photographic film is one widely used detector for diffraction patterns. When the film is directly exposed to high energy radiation, a latnt image is created proportional to the deposited energy and hence the density of the developed grains is a measurement of the radiation intensity. By measuring the optical density as a function of posititon on the exposed photographic film the diffraction pattern is recorded, and the features (spots or line segments) can be analyzed to reveal information about atomic details of the sample. While directly exposed film has very good spatial resolution and can record large areas, it suffers several drawbacks. Because film does not absorb all the X-ray quanta inciden ton it and because it has a relatively high background noise in the form of chemical fog, film is a slow or insensitive detector, i.e., high doses of radiation must be given the sample to get a readable image. The use of intensifying screens to speed up film systems has been atteempted, but it becomes difficult ot maintain intensity calibration with such screens. Film has a limited range of density linearity versus exposure, typically less than two orders of magnitude, so that widely differing intensities cannot be measured on the same piece of film. Often multiple films are used in a pack, with each film receiving a different exposure range, but the merging of the data from various films is tedious. Also, film must be processed with wet chemistry, an inconvenience. Finally, to utilize computers to analyze the data, the film must be scanned with a densitomter to convert densities to digital data, a time consuming intermediate step.
Various electronic detectors have been used to measure diffraction patterns, such as charge coupled devices, wire proportional counters, scintillators and the like. Such detectors efficiently absorb radiation quanta and have little noise, so they can be more sensitive than film, and product digital electronic data directly. However, they usually have a limited intensity range due to signal saturation or counting ratee limitations, which limits the simultaneous recording of strong and weak intensities. Also, electronic detectors have limited size so they can cover only a small area at one time. To form a complete scan the electronic detector must be moved until it has sequentially covered the entire area of a diffraction pattern resulting in additional exposure time. Recently, position sensitive detectors, which measure the position of quanta along a line instead of at a point, have been employed, but they also must be moved to cover an entire area. True area electronic detectors produced to data have not had enough active elements to be useful, either because of their small area (CCD or photodiode arrays) or poor resolution (wire grid detectors or image intensifiers). Thus electronic detectors suffer speed limitations because of their relative size and counting rate limits, and tend to have limited exposure latitude.
Given the limitations of current technologies, acquiring accurate data can often take long periods of time because of the need for high exposures, sometimes many hours or even days. More intense sources have been used to shorten this time, e.g., synchtrotron X-ray beams, but with high exposures there is often the danger of sample degradation due to radiation damage. It is thus very desirable to have detectrs which efficiently utilize all available quanta with little added noise.
One promising technology for recording radiation images is based on stimulable storage phosphors. As is revealed in U.S. Pat. No. 3,859,527 to Luckey, when certain types of phosphors are exposed to high energy radiation, such as X-rays, cathode rays, etc., they store a portion of the energy of the incident radiation. If the phosphor exposed to radiation is then exposed to a stimulating radiation, such as visible light or heat, the phosphor will emit radiation inproportion to the stored energy of the high energy radiation. Screens formed form such storage phosphors have been discussed inthe literature (J. Miyahara, et a., "A New Type of X-ray Area Detector Utilizing Laser Stimulated Luminescence" Nuclear Instruments and Methods in Modern Physics Research A246 (1986) 572578) as having very desirable properties, in terms of sensitivity and exposure latitude, for the detection of X-ray diffraction patterns from single crystal biological samples. Because storage phosphor screens efficiently absorb incident quanta and have very low background noise, hey are 10-50 times more sensitive than photographic film. They have resolutions on the order of 0.1 mm and can be made in large area formats, with millions of effective elements over a large area simultaneously integrating intensities, with no counting rate limitations. The stimulated signal is linearly related to radiation exposure over at least 5 orders of magnitude. However, it is difficult tto design analog electronics which can handle signals over such a wide range without degradation, and likewise analog-to-digital converters do not typically cover such a large signal variation. Analog compression schemes, such as logarithmic amplifiers, tend to have speed and gain limitations. Thus, while storage phosphor sysems are inherently ideally suited for diffraction intensity, it is challenging to design economical electronic systems which do not degrade the available signal.
There has been much development effort on storage phosphor systems for application in projection radiography. One of the known techniques is the use of a preliminary scan at low stimulating intensity to determine the exposure level of the latentimage on the storage phosphor screen. See U.S. Pat. No. 4,527,060 issued July 2, 1985 to Suzuki et al. Here a small percentage of the latent image is read by a low power stimulating beam, and this information is ued to optimally set the grain or scaling factor of the electronics for a full intensity final scan, insuring that no information is lost due to too high an exposure or inadequate gain. Thus it is known that multiple scans of an exposed storage phosphor screen can be used to optimally set the electronic gain in the system for a final scan.